In recent years, display devices using liquid crystal panels and display devices using organic EL panels have been under active development. These display devices are broadly classified into display devices in which only a transistor for pixel control (pixel transistor) is formed over a substrate and a scanning circuit (driver circuit) is included in a peripheral IC and display devices in which a scanning circuit is formed over the same substrate as the pixel transistor.
A display device in which a driver circuit is integrated with a pixel transistor is effective in reducing the frame width of the display device or cost of the peripheral IC. However, a transistor used in the driver circuit is required to have better electrical characteristics (e.g., field-effect mobility (μFE) or threshold) than the pixel transistor.
A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor material has been attracting attention. For example, a transistor in which a semiconductor thin film is formed using an amorphous oxide that contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier concentration lower than 1018/cm3 is disclosed (for example, see Patent Document 1).
A transistor using an oxide semiconductor for a semiconductor layer has higher field-effect mobility than a transistor using amorphous silicon which is a silicon-based semiconductor material for a semiconductor layer. Hence, the transistor using an oxide semiconductor can operate at high speed and be suitably used for the display device in which a pixel transistor is integrated with a driver circuit. Besides, manufacturing steps of the transistor using an oxide semiconductor are easier than those of a transistor using polycrystalline silicon for a semiconductor layer.
However, a problem of the transistor using an oxide semiconductor for a semiconductor layer is that entry of impurities such as hydrogen or moisture into the oxide semiconductor generates carriers and changes electrical characteristics of the transistor.
To solve the above problem, a transistor whose reliability is improved by making the concentration of hydrogen atoms in an oxide semiconductor film used as a channel formation region of the transistor less than 1×1016 cm−3 is disclosed (e.g., Patent Document 2).